re: Putter Length - How do you know?

Nothing in golf is standard. Everything comes into play if you want to get exact about it. Height, wingspan, leg length, stance, body lean, eye position over the ball, etc.
Someone mentioned that they are 6'2" and use a 35" putter. That doesn't mean shite. I'm 6'2" and run a 33" putter and grip completely down on it because I have long arms and stand with my eyes directly above the ball with a good bit of hip flex.